feat: Add GREP-0376 for Volcano scheduler backend

[xianlubird]

Summary

• Proposes Volcano scheduler backend support for Grove through native Volcano PodGroup
• Uses Volcano 1.14+ subGroupPolicy to map each Grove PodGang role into a Volcano subgroup
• Documents the end-to-end scheduling flow from PodCliqueSet to Grove PodGang to Volcano PodGroup
• Adds the Volcano 1.14+ version requirement and backend initialization capability check for PodGroup.spec.subGroupPolicy

Key design decisions

• Keep Grove PodGang as the portable scheduler API and translate it to Volcano-native resources in the backend
• Map each Grove PodGroup.minReplicas to the corresponding Volcano subGroupPolicy[].subGroupSize
• Preserve the overall workload gang requirement with Volcano PodGroup.spec.minMember
• Require Volcano 1.14+ because subGroupPolicy is needed now, and future topology-aware scheduling will also depend on Volcano 1.14+ capabilities
• Follow the direction already taken by KubeRay and Volcano Kthena for subgroup-based gang scheduling

Fixes #571

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[sanjaychatterjee]

@xianlubird Thanks for the GREP. I agree that targeting Volcano version 1.14+ is the right strategy. Overall the GREP looks good to me. I think it still needs more info regarding the TAS support. Please review PR #496 for the latest TAS updates in Grove.
@enoodle @kangclzjc Can you please review this GREP as well? Will approve once both of you give your LGTMs.

[enoodle]

Looks really good, but I think we need to address this issue: